1. Field of the Invention
This invention relates to a magnetic display wherein an image formation is performed by migrating two components of magnetic particles and non-magnetic particles in a dispersion liquid or by changing the orientation of a single component of magnetic flakes, allowing colorization of an image to be formed on a display surface.
2. Description of the Related Art
Magnetic displays currently on the market can be classified into two groups with respect to their image formation methods.
(1) One group is of the so-called “magnetic migration” type magnetic display. Two components of magnetic particles and non-magnetic particles are dispersed in a liquid and then an external magnetic field is applied thereon to shift the positions of both components to the front surface or back surface of the display, thereby forming characters or images thereof. In this case, black iron oxide fine particles or their granulated particles or the like having excellent light-absorption properties (i.e., absorbing the whole range of visible light wavelengths) are used as the magnetic particles, while the magnetic display also includes white pigments such as titanium oxide having excellent light-reflection properties (i.e., having high reflectivity to allow scatter reflections of an incident light). Therefore, a color contrast is provided using black color and white color inherent in the respective components.
(2) The other group is of the so-called “magnetic orientation” type magnetic display. Only magnetic flakes having anisotropy of shape are dispersed and suspended in a liquid, followed by the application of a magnetic field from the outside in the horizontal or vertical direction to change the orientation of magnetic flakes from horizontal to vertical or from vertical to horizontal. Therefore, a color contrast is provided using the reflection of an incident light on the surfaces of the magnetic particles at the time of a horizontal orientation and the transmission of an incident light at the time of a vertical orientation.
These magnetic displays each requires sealed containers, for enabling magnetic migration of magnetic particles or the change in polarity orientation of magnetic flakes in a dispersion liquid obtained by dispersing image-forming elements in a liquid and for keeping the display surface in a state in which the dispersion liquid is uniformly distributed on the display surface without the uneven distribution thereof. In addition, there are also several different kinds of magnetic displays according to the difference in the method for developing and maintaining the image-forming elements on the display surface. Heretofore, images formed by these magnetic displays are restricted to those of black color without exceptions as black iron oxide are used as the magnetic particles of the image-forming elements. Recently, however, there has been a demand for colorization of an image to be formed, so that several patent applications have been already filed. This invention is also provided for serving such a need. As such, the comparison between the present invention and the prior art should be only made with respect to a colorization mechanism of the magnetic display. For the accurate comparison between the colorization methods and the accurate evaluation thereof, however, the understanding of the characteristic features of the image formation method, image-formation elements, and the dispersion liquid to be used for the above magnetic display, and also of the characteristic features of the micro container to hold such dispersion liquid is essential. In the following description, therefore, the characteristics of each display will be explained prior to making a comparison between the colorization mechanisms. <Distinct features of the conventional magnetic display>
A sealed container, as shown in FIG. 7, to be used in a conventional magnetic display uses a plurality of honeycomb cells 22 which are divided by partitions thereof and sandwiched between two non-magnetic substrates 20, 21 in which at least one thereof is transparent. In addition, a dispersion liquid is filled in each of the honeycomb cells 22. In this case, the honeycomb cell 22 as a container of the dispersion liquid aims at sealing the dispersion liquid and has a function of uniformly distributing two components of black magnetic particles 24 and light-reflective non-magnetic fine particles 25 for forming an image on each part of the display face 23 without uneven distribution. Further, the dimensions of black magnetic particles 24 are selected so as to be adequate for the dimensions of the honeycomb cells 22, while the viscosity and thixotropic property of a dispersant are designed so as to be adequate for the dimensions and specific gravity of the black magnetic particles 24. That is, the black magnetic particles 24 used are those being granulated to the dimensions thereof to have appropriate liquid viscosity and thixotropic property which prevent a rapid precipitation of black magnetic particles 24 (part “D” in FIG. 7) after the image formation, where the particles 24 form an image by shifting to the front surface of the display under the application of an external magnetic field, and to allow the movements at a sufficient speed even though they are in the liquid having the above viscosity when the external magnetic field is applied to the image formation or deletion.